Thermodynamic considerations of screening halide molten-salt electrolytes for electrochemical reduction of solid oxides/sulfides

Molten salt is an indispensable electrolyte for electrochemically extracting reactive metals that cannot be obtained by a carbothermic reduction or a low-cost metallothermic reduction route. The choice of the molten salt is highly related to its thermodynamic properties, electrode materials, and the interactions of oxides/sulfides with the molten salts. Herein, thermodynamic properties of molten chlorides fitted with a solid oxide/sulfide cathode are systematically studied in terms of the electrochemical window, the exchange reactions between the oxides/sulfides and the electrolytes, and the role of cations/anions governing the deposition potential of various species in a unary molten salt or a molten-salt mixture. Thermodynamically, the choice of a molten salt for electrolysis should combine the electrochemical window of a molten salt and the in situ formed most stable oxides/sulfides possessing cations from the molten salt itself. This paper sets a guideline for screening molten salts for electrochemical reduction of solid oxides/sulfides and sheds light on the design of an appropriate salt melt for material synthesis.